The fascinating process of chemotaxis depends on a cell's ability to sense very shallow chemical gradients along its length and move accordingly. To detect gradients, cells must respond to differences in and then adapt to constant levels of chemoattractant. While adaptation is critical for directional sensing, its mechanism is unknown. Surprisingly, adaptation does not require chemoattractant receptor phosphorylation in Dictyostelium discoideum. Rather, proteins downstream of the receptor are likely to be the mediators of adaptation. Post-translational modification in response to chemoattractant of two key signaling proteins, G-protein beta/gamma subunits and Yak A, will be assessed. Novel proteins involved in the adaptation pathway will be identified by screens of insertional mutants. From a collection of all mutants, those cells with morphological defects will be isolated. Cells defective in adaptation will be identified by looking for those that maintain a pleckstrin-homology (PH) domain at the plasma membrane longer than wild type cells. In wild type cells, PH domains transiently translocate to the leading edge of the cell within seconds after exposure to a chemoattractant gradient. Very little is known about adaptation, an essential process for chemotaxis. Therefore, my studies will result in a better understanding of the processes involved in disease states in which chemotaxis is involved such as metastasis, inflammation and asthma.